This application relates to a load sense steering system with a priority valve which controls flow from a source to a steering circuit and one or more auxiliary circuits. The priority valve assures that flow needed for steering is made available to the steering circuit, and excess flow, beyond that needed for steering, is made available to the auxiliary circuit(s).
In the steering circuit, flow to and from a steering motor is controlled by a hydrostatic steering controller. The controller has a directional control valve and a positive displacement metering unit which are both operated by a rotatable steering wheel. When no steering is taking place, the steering controller is in a neutral position. When the steering controller is in neutral, the priority valve maintains a minimum standby fluid pressure at the controller, and makes the rest of the fluid from the source available to the auxiliary circuit(s). When the steering controller is operated by the steering wheel, the directional control valve, after a predetermined amount of movement away from neutral, opens a main flow control orifice to direct flow through the metering unit to the steering motor. Once the main flow control orifice opens, it can vary in size according to operator demand and steering load. The steering controller cooperates with the priority valve to insure that priority flow is provided to satisfy steering needs, and excess flow, beyond that need for steering, is made available to operate the auxiliary circuit(s).
The priority valve includes spool which is spring biased toward a priority position in which all flow through the valve is made available to the steering controller. Within the priority valve, a special pilot flow system directs a pilot flow of fluid across an orifice means to a pilot port. The pilot port in the priority valve is connected to a load sense port in the steering controller. Pressure in the pilot flow system upstream of the orifice means acts on the priority valve spool in opposition to the spring, and urges the spool in a direction restricting flow to steering and increasing flow to the auxiliary circuit(s). Pressure downstream of the pilot flow orifice means is communicated to a pilot fluid pressure chamber or cavity and acts on the priority valve in addition to the force of the spring.
The steering controller has a load sense cavity which communicates with its load sense port. The steering controller also has a neutral vent orifice which connects the load sense cavity to a reservoir when the controller is in neutral. Thus, when the controller is in neutral, a pilot flow of fluid is directed through the priority valve and the steering controller to the reservoir.
Upon actuation of the controller from its neutral position, but before the controller's main flow control orifice opens, the neutral vent orifice is substantially (i.e., at least 75%) closed. Closing the neutral vent orifice restricts the pilot flow of fluid and thereby increases the pressure throughout the pilot flow path. A dramatic pressure increase is produced in the pilot pressure chamber of the priority valve. That pressure increase assists the biasing spring to urge the priority valve spool rapidly toward its priority position.
The prior art includes load sensing steering systems for controlling flow from a source (e.g., a priority valve with a fixed displacement pump; a variable displacement pump). Such systems are described and illustrated in U.S. Pat. Nos. 3,931,711; 4,079,805; 4,167,893 and 4,043,419. Additionally, there are systems that incorporate flow control valves with pilot conduits which cooperate with directional control valve banks to maintain minimum standby pressures when the directional control valves are in neutral, to increase pressure when a directional control valve is actuated, and to dump excess flow to a reservoir. An example of such a system is shown in U.S. Pat. No. 3,815,477.
The system of the present invention represents an improvement over the systems of the foregoing art. For example, the system of the invention can provide a very fast response to steering needs, due to the dramatic increase in pressure which occurs in the pilot fluid pressure chamber of the priority valve before the main flow control orifice of the steering controller opens. Also, since the special pilot flow system originates within the priority valve of the invention, efficient pilot flow regulation and consistent response characteristics result.
The system according to the invention can either be provided with a fixed displacement pump or a variable displacement pump which has a flow compensator valve modified to incorporate a pilot flow system for controlling the output of the pump in accordance with demand.